Chlorine, hypochlorous acid, salts thereof, chlorinated isocyanuric acids, chlorinated hydantoins and other chlorine compounds (which are hereafter collectively referred to as chlorine-based oxidants) are known to achieve very effective and quick control of aquatic growth (including bacteria) even if they are used in low concentrations. Because of their high oxidizing activity, these oxidants are used in waterworks as a sterilizer, in chemical plants and cooling towers as a sterilizer or algicide for circulated cooling water, in pulp mills as a slime control agent, and in swimming pools as a disinfectant. However, the chlorine-based oxidants in aqueous solutions do not have a long keeping quality; their oxidizing ability decreases even if they are stored at ordinary temperatures, and they are decomposed even faster by the effect of heat, light and agitating force. It therefore often occurs that the intended effect of the chlorine-based oxidants is lost in a very short time.
Sodium hypochlorite may be added to the cooling water circulating in chemical plants. Since the cooling water is heated in a heat exchanger and contacted vigorously with air in a cooling tower, the sodium hypochlorite is rapidly decomposed and its concentration is decreased to a fifth to a tenth of the initial level within a period of only 30 minutes. If the decomposition rate is slow, sodium hypochlorite having a concentration of about 1 ppm can achieve the desired sterilizing effect, but because of its rapid decomposition, it must be added in a concentration of several to several tens of ppm or it must be added at short intervals, which requires much cost and labor. For disinfecting swimming pools, the residual chlorine of chlorine-based oxidants in water should be held at from 0.4 to 2.0 ppm, but the fact is that the residual chlorine is decreased at a rate of 0.4 to 0.9 ppm per hour and thus after several hours the typical residual chlorine content is almost zero. To prevent this, the chlorine-based oxidant must be added frequently, but again, this is costly and involves much labor.
The conventional chlorine-based oxidants are more or less labile, presenting problems in their use as sterilizing agents, algicides, and disinfectants. Isocyanuric acid, a cyclic amide compound, is currently used to stabilize chlorine-based oxidants used for disinfecting water in swimming pools, but its effect is not totally satisfactory. Furthermore, since the concentration of isocyanuric acid in the water is usually about 30 ppm, the resulting waste water contains an undesirably large amount of organic matter.
To solve these problems, it is reported in, for example, Japanese Patent Application (OPI) No. 26587/81 filed by the present inventors that a use of .alpha.-amino acid is useful for stability of residual chlorine to prevent the reduction of the residual chlorine with an elapse of time (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
However, the process of Japanese Patent Application (OPI) No. 26587/81 has a defect that the strong oxidizing power of the chlorine-based oxidants used causes a significant corrosive effect on iron.
Cooling water circulating through steel-made chemical plants and cooling towers contain a corrosion inhibitor to protect the areas in contact with the cooling water, but when chlorine-based oxidants are added to the cooling water as a sterilizer or algicide, it still frequently occurs that the rate of corrosion is increased. To dislodge the slime on the inner walls of chemical plants, chlorine-based oxidants are sometimes added in a concentration of as high as several hundred to several thousand ppm, and in the presence of such high concentrations of oxidants, conventional corrosion inhibitors have only a small effect in protecting the iron.